The present invention relates generally to the field of electrophotography and in particular to an optical density sensor.
Electrophotographic image forming devices optically form a latent image on a photoconductive member, and develop the image by applying toner. The toner is then transferred—either directly or indirectly—to a media sheet where it is deposited and fixed, such as by thermal fusion. In particular, it is known to successively transfer developed color-plane images from one or more photoconductive members to an intermediate transfer belt, and subsequently transfer the developed image to a media sheet for fixation thereon. Examples of an image forming device utilizing an intermediate transfer belt are the Model C750 and C752 printers from Lexmark International, Inc. Alternatively, it is known to direct a single media sheet past one or more photoconductive members, each of which successively transfers a developed color-plane image directly to the media sheet.
A problem common to all electrophotographic image forming devices, regardless of their configuration or operation, is image registration. Image registration refers to the placement of a developed color-plane image, either relative to other color-plane images or relative to the media sheet (i.e., margins, skew and the like). Numerous methodologies are known in the art for measuring and correcting registration errors. Many of these include the steps of transferring developed images comprising test patterns of various forms to a surface and detecting the developed images on the surface, i.e., detecting the presence of toner on the surface. The surface may comprise an intermediate transfer belt, media sheet or the like. In some applications, for registration purposes toner may be deposited directly on a media sheet transport belt, which normally carries the media sheets, without a media sheet being present. Regardless of the surface on which toner is deposited, one way to detect the toner is by the use of optical density sensors.
Optical density sensors are well known in the art. An optical density sensor measures the presence, and preferably the amount (e.g., in gm/cm2), of toner on a surface. This measurement may be performed indirectly, such as by sensing the differing optical properties of the surface and of toner deposited on the surface. One way to sense these properties is to illuminate the surface with a light source—preferably a collimated light source—and sensing and measuring the resulting reflections. Reflections may be generally classified as specular or diffuse. Specular reflection is reflection from a smooth surface, and tends to comprise a sharply defined beam. Diffuse reflection is reflection from a rough surface, in which a collimated beam emerges in all directions. Reflected light sensed and/or measured by an optical density sensor may include components of both specular and diffuse reflections, although one or the other may dominate, depending on the texture and other properties of the surface. The sensed optical properties are translated to toner density through calibration procedures, as well known in the art.
One known form of optical density sensor is called an integrating cavity reflectometer (also known in the art as an integrating sphere reflectometer), a representive schematic diagram of which is depicted in FIG. 15, and indicated generally by the numeral 40. The reflectometer comprises an integrating cavity 42 having a diffuse, highly reflective interior surface 44. A light source, such as a light emitting diode (LED) 46 is disposed in a collimator 48, and emits collimated light through the cavity 42 and out a view port 50, onto a surface 52. The purpose of the collimator 48 is to form a non-divergent beam of light so that all of the light that comes into the cavity 42 from the source 46 will go out the view port 50. Any light from the source 46 that directly hits the interior surface 44 will corrupt the measurement. Light incident on the target surface 52 will be absorbed or reflected (and/or transmitted if the target surface 52 is transparent). If the cavity 42 is in contact with the target surface 52, or very close to it, the reflected light enters the cavity 42, where it is reflected by the interior surface 44 until it is absorbed or strikes an optical detector 54, such as a photodiode, disposed within the cavity 42. Light striking the optical detector 54 generates a voltage and/or current proportional to its intensity, which can be sensed and/or measured. The amount of light striking the optical detector 54 is proportional to the amount reflected from the target surface 52.
The optical density sensor 40 of the type depicted in FIG. 15 is deficient in several respects. The collimator 48 is necessarily long, and difficult to integrate into a compact image forming device. In addition, a large amount of light is lost in the collimator 48, which reduces the signal-to-noise ratio of the detected light, and requires sophisticated electronics and careful calibration to obtain satisfactory results, particularly when measuring black toner, which is very absorptive and reflects relatively little light into the cavity 42. Finally, because the target surface 52 is moving (e.g., an intermediate transfer belt, media sheet or media sheet transport belt), the cavity 42 cannot contact the target surface 52, but rather must be disposed some distance above it. This distance has a strong influence on the detected signal level, since with increasing distance, more reflected light escapes and is not captured by the cavity 42. Any variation in this distance prohibits repeatable measurements; however the distance often varies as a function of age, mechanical mounting tolerances, belt motion, temperature, or even due to inconsistent belt thickness.